Since the pandemic began, like many, I’ve been making a few sourdough loaves of bread each week and periodically posting the results online. After dialing in a basic formula and method, my main focus has been on experimenting with various types of flours to explore the flavor profiles of different types of wheat while striving for a particular aesthetic (dramatic rise, open and consistent crumb). Laura Dakin and I were passing comments online recently though and she indicated that she prefers a more sour loaf and tends to modify her approach (by season, even) to accommodate her taste. When she explained what she was doing to make a more sour loaf, I gave it some thought but quickly realized that I did not understand what microorganisms in the mix were doing.
If anyone isn’t familiar with it, sourdough is essentially an ecosystem. Folks who play with sourdough generally create and keep a small amount of starter on hand, which is simply a mix of flour and water serving as an environment for microorganisms — germs and fungus in this case (bacteria and yeast, respectively). Like any environment, it has a carrying capacity, and as the populations consume existing resources more are added to prevent the extinction of the organisms inside it. For reasons apparent to me now, I had mostly been considering the yeasts in the process of bread making and neglected the roles played by bacteria, despite the fact that the bacterial population may outnumber the yeast 100:1. Further, I did not ever really understand the biology of either organism and how they impacted flavor.
In the US however, the baking industry produces $423 Billion in economic activity each year, producing about $30 Billion in annual revenue. This incentivizes funding for scientific research on the topic from both government and industry. (there are scientific journals, even!) In the civic sphere, there are also Public Libraries. And most libraries have online databases allowing citizens to access much of this research from the comfort of their own home. Thus, what I’ve gathered so far:
The Bacteria:
There are generally two types of bacteria found in a healthy sourdough ecology: AAB and LAB (and I’ll get to that). Fructilactobacillus sanfranciscensis is a famous bacterium endemic to the Bay Area and is likely present in my starter since that is where I live. Its day-to-day in a sourdough environment consists of eating carbs (glucose, from the flour, after a bit) and producing–among other things–carbon dioxide (CO2), ethanol and lots of lactic acid as waste products. Hence, it is an LAB: Lactic-acid bacteria. This Lactic acid generates a taste profile akin to yogurt (for reasons) and is responsible for that mildly tangy profile in many sourdough breads. Any AAB in a starter’s environment do their bacterial business by consuming glucose *and ethanol*, and excreting large amounts of acetic acid (and CO2 as well as other compounds). Since acetic acid is the primary component in vinegar, this is the origin of faint notes one might describe as tart or vinegar-like in various sourdough breads. Generally, LAB is dominant in most cultures. AAB generally favors a more aerobic environment (bread dough is anaerobic for the most part), competes better at cooler temperatures and has a competitive disadvantage in the the presence of lactic acid. (LABs, in this case, have a certain advantage in being able “to shit where they eat” despite conventional wisdom advising to the contrary.) Additionally, certain organic acids produced by LABs have antifungal and antimicrobial properties that may hinder the ability of AABs to compete in a shared environment. The majority of microbial research on the topic has been on LAB historically although more interest is being paid to the role of AAB lately and more will be learned as a result.
Yeasts:
The yeasts are also hungry for glucose. But being fungi, they have evolved to generate enzymes useful in decomposing their environment into more useful constituent parts. In this context, the yeasts break down the starches from the flour, converting them to sugars. And in doing so, we now have the beginnings of a synergistic relationship. By way of analogy, the yeast process the wheat (flour) to create sugar (glucose). They are exceedingly efficient and generate a surplus that the bacteria can use for their own purposes. The yeast (along with the bacteria) consume the newly liberated glucose molecules and the yeast excretes CO2 and ethanol in the proess. The latter, if you recall, is then also able to be consumed by AAB. A certain long-term symbiosis can be achieved if the bacteria and the yeast are able to gain a foothold in the starter as they will create an environment where food will be available thanks to the yeast, and interlopers will be kept at bay by the aforementioned antimicrobial properties arising from the organic acids produced by the LAB species.
It’s kind of a beautiful thing. Particularly in the early days. A problem can arise from the ethanol production of the yeast after a spell though. Similar to how AAB does not do as well in the presence of the LAB’s primary waste product (lactic acid), LAB does not do so well in the presence of the yeast’s waste product (ethanol). So the balance can be precarious, but it is a fascinating ecology to consider. Three organisms at play: the yeast helping itself and providing one basic resource (glucose) to each type of bacterium, and a second resource (ethanol) to the ‘disadvantaged’ one’ in the process; the ‘disadvantaged’ bacterium removing a substance toxic to the dominant bacterium; the dominant bacterium generating a modicum of food (ethanol) in return and contributing mightily to the defense of the environment from outside organisms. All produce CO2, which is the mechanism behind the rising of the bread (without gas being generated, you’d just bake a brick). All have distinct niches and favorable conditions and the interplay is Sourdough ecology.
None of this tells you how to actually make a loaf of bread of course, but it is useful in considering how to adjust one’s process to foster and ecology that produces the bread you want. Particularly if one can control distinct conditions in the bread making process. Information on the variable of temperature–perhaps the easiest variable to measure and control for any mix–are below.

credit: JMonkey at http://www.thefreshloaf.com/
If the above is true–and why would you NOT believe something from someone named JMonkey you found on the internet?–at ranges outside of 65′ and 74’F, LAB populations can out-compete yeast.
JMonkey jokes aside, the graph holds up, supported by a paper published by the Journal of Applied and Environmental Microbiology: Modeling of Growth of Lactobacillus sanfranciscensis and Candida milleri in Response to Process Parameters of Sourdough Fermentation. (authors: Michael G. Gänzle, Michaela Ehmann and Walter P. Hammes):

In the weeks to come, I’ll likely try to put this to practical use.
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